Catalytic reforming of naphtha fractions has long been regarded as an attractive means for providing gasoline blending components having high octane numbers. The demand for such blending components has become critical as the use of organometallic octane appreciators, such as lead alkyls, has diminished in response to environmental constraints.
Catalytic reforming generally involves a complex series of hydrocarbon reactions, employing a hydrogenation-dehydrogenation catalyst, wherein a substantially paraffinic and/or naphthenic naphtha fraction of petroleum variously undergoes dehydrocyclization, dehydrogenation, isomerization and hydrocracking to provide a mixture substantially comprising aromatic, olefinic, naphthenic and isoparaffinic hydrocarbons. Such mixtures possess suitably high octane numbers and generally excellent blending characteristics. The reforming reactions are, on balance, endothermic and are generally conducted with serial flow through a plurality of reactors at elevated temperatures in the presence of hydrogen, with provision for heating between reactor stages.
Noble metal catalysts generally are effective in the promotion of the spectrum of chemical conversions characteristic of reforming. Such metals are quite effective at relatively low concentrations, for example, 1 weight percent or less, when extended upon a support material. Suitable support materials must possess sufficient surface area to provide an adequate base for dispersion of the noble metal. Further, the support must possess pores having dimensions large enough to accommodate the chemical reactants to be acted upon. In general, an adequate surface area for most reactants will be in excess of at least about 100 square meters per gram. For most reforming operations, a generally preferred noble metal is platinum and a preferred support material is alumina. Platinum is often used together with a second metal. Alumina is often modified to increase acidity of the support, as by contacting with a halide-affording material.
Catalysts comprising platinum, such as platinum dispersed on an alumina support, are generally employed in the reforming of naphthas because of their overall excellent performance, despite high cost, and high selectivity toward the production of aromatic hydrocarbons boiling in the gasoline range. Maintenance of platinum catalyst activity and selectivity can be improved by the use of a second metal. A preferred second metal is rhenium, whose use is particularly described in U.S. Pat. Nos. 3,415,737, 3,496,096 and 4,176,088.
Since most of the desirable chemical reactions, including hydroisomerization and dehydrocyclization, require acidic conditions, it is necessary to provide an effective level of acidity in the catalyst. This is generally accomplished by the introduction of a halogen, usually chloride, which is held on the surface of the catalyst support material together with the catalytic metals. The halogen material is replenished throughout the catalytic cycle by addition, usually in the form of an organic halide, to the hydrocarbon feedstock. Control of the halide concentration on the catalyst by control of the water-halide ratio in the feed is well known.
One method for increasing the activity of the reforming catalyst, particularly in terminal reactors, comprises significantly increasing the chloride content in the catalyst, for example, to a level substantially above 1.0 weight percent. However, this approach is often not feasible because loss of catalyst surface area with continuing on-stream time leads to loss of chloride from the catalyst, thus requiring an excessive make-up rate for chloride and causing excessive downstream corrosion.
Although much attention has been given to this problem, there is a continuing need for an improved support material, possessing suitably high and stable surface area and pore size characteristics. A desirably improved support will exhibit a substantially diminished rate of change in desirable surface parameters such as surface area and pore size upon continued exposure to high temperatures. Such increased stability of the support material will effectively reduce the tendency toward a gradual loss of halide promoter.